Abstract Valence states of molybdenum and iron ions and their cationic distribution on both octahedral (B) and tetrahedral (A) sites of the spinel structure of molybdenum-substituted magnetites, Fe 3− x Mo x O 4, obtained by two ways (samples synthesized by w et process or ground samples synthesized by the ceramic route) and which are oxidized in cation deficient spinels, have been performed by derivative thermogravimetry (DTG), X-ray diffraction, IR and XPS spectrometries. It has been demonstrated that in the case of the samples synthesized by wet processes, the Fe 2+ B, Mo 3+ B, MO 4+ B, Fe 2+ A, Mo 4+ A ions are successiv ely oxidized into Fe 3+ B and Mo 6+ A ions below 530°C. For the samples synthesized by the ceramic method, only the Fe 2 B, Mo 3+ B, Mo 4+ B and Fe 2+ A oxidations have been observed. In these both cases, a quantitative analysis based on the discrepancy in reactivity of these different oxidizable cations has been used to determine the distribution of cations between the sublattices. As a result of the oxidation of all the oxidizable cations, the molybdenum defect ferrites may have a very high number of vacancies. These may reach 0.71 for x=0.5. The possibility to oxidize the Fe 2+ B and Mo 3+ B ions selectively, permits on the one hand to confirm the desummations of the DTG curves, and on the other hand to study the oxidation kinetics of these two ions. It has been also possible to show that the profile of the experimental DTG peaks obtained after desummation is comparable to the DTG peaks calculated from the isothermal kinetic parameters. Finally, the study of the cationic distribution of the oxidized phases has shown that the Mo 6+ ions, initially in tetrahedral coordination can remain in octahedral site as the octahedral sites are also occupied by Fe 2+ or Co 2+ ions.